It is common knowledge that in a Diesel engine injection of the fuel occurs when the piston of a given engine is in the zone of top dead center (TDC). The initial injection is timed to occur from slightly before TDC to shortly after TDC, dependent upon engine speed; in general, injection occurs at an earlier point at a higher rpm than at a lower rpm. Though the time period required for fuel to be fed from the pump to the jet is largely constant and independent of the rpm level, the time period between fuel pump output and its combustion in the engine varies corresponding to the rpm level.
This variation in the cycle of injection timing is compensated for by means of an injection timing adjustment. Though the primary task of the adjustment device is injection timing modulation, the residual capacity of the device serves, dependent upon the combustion requirements of a given engine, to improve the fuel consumption, the HP and torque output, the smoothness of operation, and/or the exhaust gas quality. For example, when the injection timing is closely matched to the desired exhaust gas compositions, in order to decrease polluting by-products of combustion, then it is necessary to decrease the degree of advancement in the injection timing cycle at an rpm above the middle rpm range of a given engine, in order to prevent excessive roughness in the operation of the engine.
A known fuel injection pump has at least one pump piston powered by a cam drive assembly which is rotatable for the purpose of injection timing adjustment. The rotation is imparted by a servo mechanism subject to rpm-dependent hydraulic pressure and against the force of two separate return springs acting in parallel.
The use of two springs makes it possible, for a first approximation, to decrease the slope of the characteristic curve in the upper rpm domain, i.e., that curve which describes the injection angle as a function of engine speed.
The injection timing is thus set primarily to attain the desired emission characteristics of the engine. This setting leads, however, to decreased smoothness in running of the engine, above the middle rpm range of a given engine.